so I just placed an order on OBX LT Headers. Should I change my mid pipes size to 2.5" or keep it stock? will it make a big difference in sound, performance?

btw I have MRT V1.0 Axle back I have no idea if its even possible to install 2.5" pipes with that.

any suggestions?

It won't really make a difference with performance, however it will in sound. I first had MRT V1 then I got ARH LT headers and high flow cats and it sounded kinda weird, a little too raspy for my liking. So I bought a pair of SS LS3 mid pipes and it made the sound a lot better. If you're going to get mid pipes definitely try and get a pair of SS mid pipes because it fits great and it will be much cheaper. One thing to note, is it needs to be from an LS3 engine NOT an L99. The L99 mid pipes are different and have a lot of extra crap that isn't needed.